Formal charge:

When do you use it? How is it calculated?

Formal charge is used in chemistry to evaluate the distribution of electrons in a molecule or ion. It helps to determine the most stable Lewis structure by assigning formal charges to individual atoms within a molecule. Formal charge can be used to predict the reactivity, stability, and bonding patterns of molecules.

To calculate the formal charge of an atom, you need to follow these steps:

1. Determine the number of valence electrons for the atom. The valence electrons are the electrons in the outermost energy level of an atom.

2. Count the number of electrons the atom has in the Lewis structure. Each lone pair (non-bonding pair) of electrons is counted as two, and each shared pair (bonding pair) between atoms is counted as one.

3. Subtract the number of electrons from step 2 from the number of valence electrons in step 1. The result is the formal charge.

The formula to calculate the formal charge is as follows:

Formal charge = Number of valence electrons - Number of lone pair electrons - 0.5 * Number of bonding electrons

After calculating the formal charges of all the atoms in a molecule or ion, the sum of the formal charges should equal the overall charge of the molecule or ion, if any. If there is a charge, it should be accounted for by the formal charges on the atoms.

By considering the formal charges, you can determine which Lewis structure is the most stable one and understand the electronic distribution within the molecule or ion.